An electromagnetic induction device, such as a transformer or a reactor, has a magnetic circuit, the magnetic core, typically made of laminated iron of grain oriented steel or ribbons of amorphous steel, and an electrical circuit, windings, usually made of aluminium or copper wire. A third component, the housing or tank, is necessary in case of fluid filled electromagnetic devices.
The magnetic core comprises one or more limbs. The number of limbs depends on the number of electrical phases to be connected to the electromagnetic induction device. Windings are arranged around each limb.
Varying current in the windings create a magnetic flux in the magnetic core. The magnetic flux in the magnetic core is the primary magnetic flux. Magnetic flux may also be created in other components of an electromagnetic induction device. The windings, for example, may create a magnetic flux, called the leakage flux or leakage magnetic flux, which produces non-desirable losses while passing through the side walls. Such devices typically comprise magnetic shunts, which are metal plates arranged between the windings and the side walls of the housing to reduce the losses.
In the event of more than one limb, the magnetic circuit also comprises yokes, namely an upper yoke and a lower yoke. The yokes mechanically connect the limbs at their ends. A return path which enables circulation of the primary magnetic flux in the magnetic core is thereby obtained.
For large electromagnetic induction devices, such as high voltage rated power transformer, the magnetic circuit may comprise side limbs to reduce the yoke dimension to thereby reduce the overall height dimension of the electromagnetic induction device. Due to transport limitations, e.g. bridge heights, this may be essential to be able to transport an electromagnetic induction device to the site of installation.